Here, all you have to do is substitute (7) for (y) in the equation. You can do this because all coordinates are of the form (x,y), and any number that is in the place of (y) in this format can be seen as equal to (y).
The working is shown below:
5x - (7) = 3
5x = 10
x = 2
As shown in the working above, x = 2 when y = 7.